Scab of Winter Wheat

May 24, 2002

The very wet weather has kept many of you from slogging through the fields on wheat scouting missions; however, the weather has been very good for scab to develop. Myself, I have not slogged through any fields this week, either, but now is the time to look for head scab in fields.

Scab (also called head scab or head blight) is caused by fungi in the genus Fusarium. Principally, the pathogens are Fusarium graminearum, F. avenaceum, F. culmorum, and F. nivale. These Fusarium species are all asexual forms that produce only conidia. The sexual stage is known as Gibberella zeae. Scab-infected wheat seed that is planted may develop root rot, as well. These Fusarium fungi are ubiquitous and can, unfortunately, also cause a seedling blight and stalk, ear, and root rot of corn.

Symptoms

Typical symptoms of scab infection occur soon after flowering. Spikelets appear bleached, with a light straw color, and will ripen prematurely. The heads may be sterile, or, if kernels are produced, the berry will be small, shriveled, and off color. You will see the typical white, bleached, scabbed heads scattered throughout a field. The Fusarium spores are produced during warm, wet weather and only infect wheat flowers that are blooming. So, it is not unusual to see only half of a wheat head infected or scattered spikelets on a head infected. It is just dependent on the blooming pattern of the wheat variety and the presence of the spores. If you carefully examine the bases of the glumes on an infected head, you may see the diagnostic pink-to-salmon-colored Fusarium mycelium.

Disease Cycle

Scab fungi survive and reproduce in and on the soil as spores and mycelium, or in sexual fruiting structures (perithecia). The asexual spores (conidia) are produced during warm, moist weather. The sexual spores (ascospores), produced in the perithecia, are discharged into the air during warm, moist weather in the spring and early summer. Wind and splashing rain carry the ascospores and conidia to the wheat spikelets. The spores germinate in a film of moisture and first invade the flower parts. Infections are most frequent and serious at anthesis. Scab symptoms will develop within 3 days after infection, when temperatures range between 77 and 86 deg F and moisture is continuous.

Planting scab-infected seed generally results in very poor stands. Scabby kernels are often dead or else germinate weakly. If a sprout manages to emerge from the soil, it frequently decays before it can become established.

Wheat seedlings may also be infected. These infections result primarily from seedborne mycelium and spores. When the soil temperature is above 60 deg F, seedlings from clean seed may also become infected from mycelium in decaying crop residues on or in the soil. Note that head infections occur independently of seedling blight and root rot because the fungus cannot grow for any distance within the cereal plant.

Mycotoxins

The growth of the Fusarium fungi in infected grain produces serious mycotoxins that cause muscle spasms, acute vomiting, nausea, dizziness, diarrhea, and soreness in humans, young chickens and ducklings, pigs, dogs, horses, and other nonruminant animals with simple stomachs. In swine feed, 3% or more of scabby grain causes vomiting, and then the swine refuse to eat the mixture. Cattle, sheep, and mature poultry (except pigeons) do not react to scabbed grain. Because one of the mycotoxins involved has some estrogenic activity, scabby grain should not be fed to breeding animals. The mycotoxins apparently remain stable for years in stored grain.

Heavily scabbed wheat kernels are generally very lightweight and easily removed by modern cleaning equipment. In other words, turn the blower up on the combine.

University Recommendations for Management of Scab

1. Rotate small grains and corn with legumes, allowing at least a one-year break in cereal, grass, or corn cultivation. Plant small grains as far as possible from old cornfields.

2. Sow only plump, small-grain seed that has been thoroughly cleaned to eliminate all lightweight seed and then treated with a protective, broad-spectrum fungicide or fungicide mixture. Proper seed treatment controls seedling damage from infected seed but will not control the head-blight or scab, foot-rot, and stem-blight phases of the disease.

4. Delay the sowing of winter cereal grains until the temperature is 60 deg F or below, to reduce the chances of severe seedling blight. Sowing spring cereal grains early tends to reduce losses from seedling blight, crown rot, and head blight.

5. Where feasible, plow under cleanly and deeply all infected stubble debris and straw of small grains and weed grasses, cornstalks, and rotted ears. Complete coverage of crop residues helps reduce head-blight infections. Sanitation is most effective when it is done on a communitywide basis. Manure containing infected straw or cornstalks should not be used for top dressing.

6. No highly resistant varieties of wheat, oats, barley, or rye are available. Some varieties are infected less frequently, apparently due to physical barriers to the infection of florets and spikelets. Not all differences in the incidence of head blight among wheat or barley varieties growing in adjacent fields are genetically based. The time of anthesis and prevailing weather conditions at flowering can also influence the development of scab.

7. Store grain at a moisture level of less than 14% to prevent the growth of the fungus and the possible production of additional mycotoxins.--Suzanne Bissonnette